1. Field of the Invention
This invention relates to inorganic pigments used to sensitize dye precursors in so-called "carbonless copying record" systems, in particular those systems which employ an electron donor-acceptor reaction between particles of a solid electron acceptor distributed on image-receiving substrates and an organic solution of a colorless or substantially colorless basic dye precursor on transfer substrates. The invention is directed to the provision of an improved reactive inorganic pigment which is capable of developing solutions of basic dye precursors such as crystal violet lactone which normally form fugitive images when sensitized with prior art acidic inorganic pigments. In particular, the invention is concerned with a method for processing dioctahedral montmorillonite crude clay to produce novel sensitive inorganic pigments for use in record material, the novel pigments being members of the class of materials referred to as "reduced charge montmorillonites" or "reduced charge bentonites."
Several varieties of commercial carbonless copying paper utilize the chemical reaction between particles of a solid acid and a colorless or substantially colorless basic chromogenic compound to form a printed image. In the early stages of the development of cabonless copying paper systems, the image-developing receiving members were provided by applying an aqueous coating composition containing a paper coating adhesive and particles of attapulgite clay, a unique clay material. These CF members were assembled with transfer members coated with an encapsulated oily solution containing both a primary dye precursor, crystal violet lactone (CVL), and a secondary dye precursor, benzoyl leucomethylene blue (BLMB). Reference is made to U.S. Pat. No. 3,330,691 to Adams et al. Upon application of printing pressure to the assembly, the capsules ruptured and their contents were adsorbed on the clay particles disseminated on the CF sheets. As a result of an electron donor-acceptor reaction between acidic sites on the particles of attapulgite clay and the CVL, a dark blue mark was formed essentially instantaneously. This image faded rapidly and was replaced when the more slowly reacting BLMB was converted to colored (blue) form by a combined hydrolysis-oxidation mechanism. One of the problems with this type of record material was that the receiving sheets tended to lose sensitivity to the dye precursors during storage. Another was that the receiving sheets lacked the whiteness and brightness of conventional printing paper. Furthermore, aqueous suspensions of attapulgite clay were highly viscous and conventional high speed paper coating techniques could not be used.
The necessity for using both primary and secondary dyes also had obvious defects. The use of the secondary dye added to the cost of manufacturing the carbonless copying paper and it necessitated careful balancing of the relative proportions of primary and secondary dyes to assure that an image was present during the transition between fade of the CVL and full development of the BLMB. However, even with judicious selection of proportions of the dye precursors, color changes were evident throughout the life cycle of the developed sheets.
Some but not all of the drawbacks of the attapulgite pigment system have been obviated by sensitizing sheets with a pigment obtained by leaching a dioctahedral montmorillonite clay with mineral acid. The acid treatment of the crude clay increases the intensity of the image developed by CVL and it converts the clay from a material having rheological characteristics completely unsuitable for paper coating to a product which can be coated at higher solids than the attapulgite pigment. Acid-treated dioctahedral montmorillonite, exemplified by the product supplied under the tradename "Silton," has supplanted attapulgite clay as the most widely used inorganic pigment sensitizer for carbonless copying paper. It is well known, however, that CVL produces a fugitive image when developed on acid-treated dioctahedral montmorillonite such as Silton and, as shown in U.S. Pat. No. 3,622,364 and U.S. Pat. No. 3,753,761 to Sugahara et al., BLMB must be used along with the CVL. Even when using this dye precursor combination, there is a considerable change in color in the printed sheets since CVL fades from deep blue to green when it is developed on Silton. Certain solutions of carbinol dye precursors have been shown (South African Patent No. 72/1193 to Baxter) to produce essentially stable images on montmorillonites leached with sufficient acid to increase the hydrated silica content and to destroy partially the mineral lattice. Examples of such solutions are those obtained by dissolving Michler's hydrol, derivatives of Michler's hydrol such as the benzyl ether and morpholino compounds in polar solvents of low volatility. According to the teachings of the South African patent, the dyes are stabilized by penetration of the solution into the lattice of the mineral structure. To the best of our knowledge, however, acid-leached montmorillonites capable of producing stable images with CVL are unknown and, further, the rheological properties of coating mixtures prepared using sufficiently high contents of such pigments to give adequate imaging performance are only marginally adequate for use with conventional high-speed blade coating techniques for coating paper webs.
A wide variety of other inorganic pigments has been proposed for use in carbonless copying paper and many have been reported to produce images of high intensity when placed in absorptive contact with solutions of basic chromogens such as CVL. Among such pigments are: hydrothermally treated kaolin clay (U.S. Pat. Nos. 3,223,546 and 3,226,252 to Hemstock; U.S. Pat. No. 3,723,174 to Swanson et al.; U.S. Pat. No. 3,736,285 to Miller); acid-leached calcined clays such as kaolin, montmorillonite or attapulgite (British Patent No. 1,307,319); clays such as the aforementioned calcined above 200.degree. C., preferably between 500.degree. to 700.degree. C. (French Patent No. 1,589,266 to Hayashi et al.); and lattice-expanded kaolinite (U.S. Pat. No. 3,520,719 to Horton). Insofar as we are aware, none of these pigments reacts with CVL to produce images that are sufficiently resistant to fade to be useful with transfer sheets prepared with CVL without secondary dye precursors. In this regard, it is noted that binders and dispersants used in paper coating compositions (coating colors) generally adversely affect pigment sensitivity. Therefore, pigments which are promising as image-formers when tested in cake form may be entirely unsatisfactory when formulated into coating colors and coated on paper.
An increasing amount of the carbonless copying paper that is being supplied to the market is based on the use of a solid acidic resin, usually a phenolic resin. The phenolic resins, although expensive, may be used at low levels since they are very reactive and are normally made into coating mixtures with large quantities of kaolin coating clay, hence the designation "K-P" or kaolin-phenolic system. The rheological properties as therefore good and high solids blade coating is possible. Certain K-P sheets produce images of acceptable stability with CVL and thus BLMB or other slowly reacting secondary dyes may be excluded from the system. However, the resins are stringent in their requirement of the precursor solvent, which must also be a solvent for the resin. The original and most suitable solvents in this regard were chlorinated aromatic hydrocarbons. A measure of the stringency of the requirement on solvents is the fact that since the chlorinated aromatics were banned for use in this application, no equivalent substitutes have been discovered. Solvents commonly used in current practice are mixtures of aliphatic, alicyclic and/or aromatic hydrocarbons, which perform adequately in this respect but are not as suitable as the chlorinated aromatics. Furthermore, the resins become oxidized and polymerized to give quinones and other products, and the coated sheets generally turn yellow and develop an offensive odor on storage. In addition, they may cause frequent washups on offset presses due to their partial solubility in alcohol-containing fountain solution, resulting in resin buildup on the press. Furthermore, the resin-coated sheets may not be used as broke on a paper machine due to the presence of the resins.
2. Prior Art
The term reduced charge montmorillonites, as described in the literature, embraces a series of montmorillonites of progressively lower charge, as determined by lattice expansion characteristics and quantity of exchangeable monobasic cations. Members of the series are obtained by exchanging exchangeable cations of dioctahedral montmorillonite with various amounts of lithium cations, followed by mild heat treatment to collapse the mineral lattice and fix the exchanged lithium cations (render lithium ions non-exchangeable). A rationale for the development of the unique properties of reduced-charge montmorillonites has been provided in the literature, specifically by G. W. Brindley and G. Ertem, CLAYS & CLAY MINERALS, 1971, Vol. 19, pages 300 to 404; G. Ertem, ibid, 1972, Vol. 20, pages 199 to 205; and R. Calvet and R. Prost, ibid, 1971, Vol. 19, pages 175 to 186. It appears that on heating, part of the lithium migrates from the interlayer space into vacancies in the octahedral layer of the mineral lattice, whereby the negative charge on the mineral lattice decreases by one unit for each lithium ion which migrates. Since in a montmorillonite the lattice possesses a negative charge which balances the positive charge of the interlayer exchangeable cations, this negative charge is thereby reduced by those lithium cations which become "fixed" to the lattice. Hypothetically, this reduction of charge reduces the electric field in the interlayer region, and thus reduces the tendency of the mineral lattice to expand by adsorption of highly polar liquids into that space, but increases its affinity for less polar liquids. In addition, part of the lithium is said to react with lattice hydroxyl groups to produce protonic acidity on the surface of the mineral and in the inter-layer space. Evidence for this is given in the literature cited. To the best of our knowledge, reduced charge montmorillonite has no practical utility. Insofar as we are aware, the material as produced for scientific evaluation is in the form of chunks which would not be useful as a coating pigment and we know of no suggestion for employing such material in a form useful for such purpose.
U.S. Pat. No. 3,674,521 to Noble discloses that treatment of kaolin clay with ions such as lithium ions followed by a mild heat treatment results in a kaolin clay coating pigment having reduced viscosity. Included in the disclosure is a description of the use of such a process with a kaolin clay including an unspecified amount of a montmorillonite impurity. The patent to Noble is not concerned with the provision of sensitized coatings for developing dye precursors. At any rate, we have found that such treatment of kaolin clay does not result in a pigment useful for sensitizing recording material for printing with CVL solutions.